It has been known for many years that fingerprints can be used to check a person's identity. For the checking, a current fingerprint is recorded from the person whose identity is to be checked and this is compared with previously recorded reference fingerprint data. The checking can concern a verification of the identity of the person. In such a case, the current fingerprint is compared only with reference data for the person that the person whose identity is to be verified is purporting to be. The checking can also concern an identification of the person's identity. In such a case, the current fingerprint is compared with reference data for several different persons in order to determine whether the current fingerprint comes from any of these persons.
There are different types of sensors for recording fingerprints. The sensors can for example be optical, thermal, pressure-sensitive or capacitive. Traditionally, the sensors have had a surface with a size that makes possible the recording of a complete fingerprint. Particularly with regard to capacitive sensors, that are manufactured from silicon, the cost of a sensor is, however, greatly dependent upon the size of the surface of the sensor. There is therefore a desire to be able to make the sensors smaller.
EP 0 813 164 describes a thermal sensor that only allows a part of a fingerprint to be recorded. In order to record a complete fingerprint, the user passes his finger across the sensor which records images at a high frequency. The images, that partially overlap each other, are then combined into an image of a complete fingerprint. The identity check is carried out in the usual way on the basis of the complete fingerprint.
This sensor has the advantage that it has a smaller sensor surface than sensors that record a complete fingerprint with a single image. The disadvantage, however, is that it is more difficult to use, as the user must pass his finger over the sensor at a relatively even speed in order for the image to be clear. If the user passes his finger over the sensor slowly or even stops, the result will be impaired as the sensor is based on temperature differences that are evened out immediately if the finger is held still.
WO 00/49944, that was filed by the Applicant of the present application, describes the use of a small sensor that records a partial fingerprint, that is only a part of a complete fingerprint. According to this application, a reference fingerprint image is recorded using a traditional “large” sensor or using a small sensor with the images combined into an image of a complete fingerprint. For the identity check, on the other hand, only an image of a partial fingerprint is recorded. This partial image is compared with the considerably larger reference image in all possible relative positions in order to check whether the partial image corresponds to any partial area of the reference image and can thereby be assumed to come from the same person as the reference image.
This method makes it possible for the user to hold his finger still on the sensor during the identity check. The comparison of all overlapping positions requires, however, a certain processor capacity and can take a relatively long time. In addition, an essentially complete reference fingerprint must be stored. This can be a disadvantage, for example when the check is to be carried out on a smart card which normally has limited processor and memory capacity.
In other known methods for checking identity using fingerprints, a complete fingerprint is, however, not normally stored as reference fingerprint data, but instead the most relevant information is extracted from the fingerprint and saved as reference fingerprint data. For example, different partial areas of an image of a fingerprint can be saved as reference data.
WO 01/84494, that was also filed by the Applicant of the present application, describes in addition how the identity check can be divided between a smart card and a processing unit in order to minimize the operations that need to be carried out on the smart card and that therefore require processor capacity. More specifically, the reference data is divided in this instance into a public part, that can leave the smart card, and a private part, that cannot leave the smart card. The public part contains a partial area and coordinates that indicate the location of additional partial areas that are to be found in the private part. To carry out an identity check, a current fingerprint is recorded for the person whose identity is to be checked. The public partial area is transmitted from the smart card to the processing unit in which it is compared with the current fingerprint in order to check whether it matches anywhere in this. If such is the case, the relative positions (translation and rotation) of the reference data and the current fingerprint are determined. The coordinates in the public part of the reference data are hereafter used to determine the partial area of the current fingerprint that is to be matched against the partial areas in the private part of the reference data. These partial areas of the current fingerprint are sent to the smart card where the identity check is concluded. As an alternative to the public partial area, a set of special characteristics, so-called features, can be stored in the public part of the reference data and used in order to determine the relative positions of the reference data and the current finger-print.
If a small sensor, that is a sensor that only records a partial fingerprint, is used for this application, the problem arises that it can happen that the user places his finger on the sensor in such a position that only some of the partial areas or none of these at all lie within the sensor surface area. If this occurs, the identity check will not succeed due to the fact that there is an insufficient number of partial areas to compare. The user must then carry out a new attempt with his finger in a new position. A number of attempts can be required and this is of course inconvenient for the user.
SE 0102376-1, which was filed on 29 Jun. 2001 by the same Applicant as in the present application and which was thus not publicly available at the time of filing the present application, proposes a solution to this problem which consists of checking how well the reference data is aligned with the current fingerprint. More specifically, on the basis of where the public partial area is located in the current fingerprint and using the coordinates for the private partial areas, it is possible to work out whether the private partial areas are located within the sensor surface area and thus whether the identity check has the possibility of success. If such is not the case, an indication can be given to the user regarding how he or she is to move his/her finger in relation to the sensor in order to improve the alignment.